(1) Field of the Invention
The present invention relates to exhaust emission control system and method, particularly relating to an exhaust emission control system for an internal combustion engine which facilitates warming a catalyst up by causing a heater to function, which catalyst has the heater attached thereto.
(2) Description of the Related Art
Generally speaking, a catalyst used for controlling exhaust emission cannot be activated and thus the necessary purifying function cannot be obtained unless the temperature thereof reaches a predetermined value. The catalysis used for the internal combustion engine is heated by exhaust gas provided therefrom. However, in a case where the internal combustion engine is started when the ambient temperature is not high enough, the temperature of the exhaust gas is thus not high enough, further the engine has not been warmed up yet and combustion efficiency is thus not yet high. Therefore, purifying the exhaust gas by the catalyst is difficult in that case.
The Japanese Utility Model Laid-Open Application No. 49-124412 discloses an exhaust emission control system of a related art, which system has a heater for heating a catalyst, the heater heating the catalyst, a temperature sensor then detecting when the temperature of the catalyst reaches a temperature in an activating temperature range, that is, a range in which the catalyst is activated, the heater then stopping the heating of the catalyst in response to an output from the temperature sensor.
Such a system of the related art is needed to heat a catalyst appropriately so as to reach a temperature in the activating temperature range in any condition, a high detecting accuracy and a very fast response being thus required for a temperature sensor to be used for detecting the temperature of the catalyst in the exhaust emission control system in the related art.
For the purpose of the high detecting accuracy and very fast response in the temperature sensor, the temperature sensor is located on the back of the catalyst, the temperature on the back of the catalyst reaching an extremely high temperature, so as to expose the sensor directly to the exhaust gas, in the system of the related art. Further, a thermocouple is employed for the temperature sensor, such a thermocouple having an excellent response time in the system. However, such a thermocouple, although it has an excellent response time, is inferior in view of durability thereof, in comparison to another temperature sensor such as a thermistor. Particularly in the above-mentioned extreme circumstances, a problem such as the snapping of a wire may occur in the thermocouple accordingly.
A problem occurring in the temperature sensor results in continuous heating of the catalyst by the heater, causing further problems such as the catalyst being excessively heated, or damaged. Another problems in which the catalyst is not heated so as to be activated sufficiently and the sufficient purifying of the exhaust gas cannot be achieved may occur as a result of the temperature sensor failure.